This invention relates to measurement of spectral characteristics by transmission of a coherent beam of radiation, such as infrared light, to measure atmospheric properties.
Lasers are ideally suited for making a class of long range measurements involving the transmission of an optical beam from a source to a distant target, the scattering of the illuminating radiation from the target, and the detection of the scattered energy by a receiver (or multiple receivers coincident with or in the vicinity of the transmitter. Information is obtained by measuring the return signal strength and other parameters, such as the round-trip travel time, Doppler shift of the returned radiation, and polarization changes. The information includes target properties (size, distance, velocity, and range-resolved rotational velocity) as well as properties of the medium through which the optical beam is traveling to and from the target. Information pertaining to the target may be grouped under the general category of laser radar (ladar) while the measurements pertaining to the optical medium are generally grouped under remote sensing (lidar).
For example, with a ground-based source and airborne target, measurement of signal strength yields information about a target's physical properties. Measurement of the round-trip travel time to and from the target provides the target's range measurement. Determination of the frequency shift of the return radiation provides a direct measure of the target's velocity relative to the source. This is the basis for an optical radar system.
If the illuminating source is airborne and the target is the ground, one may also infer the presence of absorbing atmospheric species along the optical path at the transmitted wavelength by measuring the strength of the back-scattered signal. This is the essential technique for remote sensing of chemical species, such as airborne pollutants. This can be achieved by standard path-integrated or range-resolved differential absorption measurements using pulsed laser sources. Path-integrated systems rely on topographical returns of an echo signal, while range-resolved sensors use aerosol back-scattering.
A pollutant can be identified, for example, from its spectrally resolved absorption signature. The absorption measurements are generally performed in the infrared atmospheric windows (3-5 microns and 8-12 micron wavelength) although the techniques described below will provide equivalent benefit in the visible and near infrared region (0.4.fwdarw.2.0 micron wavelength). The measurements can be conducted using standard direct detection or, in the infrared region, much more sensitive, coherent (heterodyne) detection.
The utility of these lidar and ladar measurements depends upon the capability to operate at long range. At a given range, the signal-to-noise ratio determines overall system parameters such as target size, reflectivity and surface quality and the system's transmitted energy, transmitter and receiver aperture sizes, and detection sensitivity.
One strives to achieve single photon detection sensitivity to optimize system performance. In the visible and near-infrared regions where thermal-noise sources produce little competing signal, direct detection, which depends upon the return signal energy or power and is proportional to the square of the return electric field amplitude, affords single photon detection capability. In the longer wavelength, infrared region, where thermal radiation-induced noise can mask the return signal strength, coherent detection, which depends upon the amplitude of the return signal electric field and requires an additional optical source to serve as a local oscillator, delivers single photon detection capability. Although spectral operating range, hardware complexity and desired sensitivity dictate the detection mode, both methods take advantage of the laser beam's high brightness and directionality which is a result of the laser's beam spatial coherence.
Temporal coherence causes a problem for both detection modes by producing a speckled pattern in the receiver plane as a result of interference among electric field contributions scattered from a rough (diffuse) target surface. The surface may provide specular or diffuse reflection of the illuminating radiation depending upon the scale size of the surface roughness. When the scale size for surface irregularities is small compared to the illuminating wavelength, the return is specular. For surface roughness scale sizes comparable to or larger than the illuminating wavelength, the scattering is diffuse and speckle is produced.
Speckle degrades system performance by adding a random, pulse-to-pulse fluctuation to the return signal electric field. The fluctuation is a result of small changes in the optical path length (comparable to the wavelength of the illuminating radiation). Such changes in the optical path length may be produced by target or source motion, or by atmospheric fluctuations appearing within the optical transmission path between successive pulses of the laser radiation. The pulse-to-pulse speckle generated fluctuations increases the variance in the return signal measurement which serves to decrease the effective signal-to-noise ratio and, thereby, decrease the measurement precision.
For direct detection systems which measure the square of the return signal electric field and are insensitive to its phase, speckle can be accommodated (and the resulting variance reduced) by increasing the receiver aperture to collect a number of speckles simultaneously. The resulting detector output represents an average over the individual speckle intensities and more closely represents the mean power scattered from the target. This technique, however, is not applicable to coherent detection since the output signal depends upon the electric amplitude and phase. Well known analyses have shown that the variance in the coherently detected signal does not diminish as the aperture size decreases. In the past, the lack of an effective means of speckle-induced variance reduction have discouraged the use of coherent detection and have made unavailable its significantly greater sensitivity in the infrared region as compared to direct detection.
Systems employing coherent illumination and either direct or coherent detection methods can offer the possibility of achieving the long ranges which are desirable in many surveillance type functions providing that the aforementioned interference due to speckle can be overcome. The invention below provides a method to reduce the speckle-induced fluctuations for both detection modes.